PEOPLE STOP RAGING! It is obviously Obito because the freaking title is "Obito Uchiha"! My theory on how Obito survived is that when Obito got hit by the boulder, only a tiny bit of his face was actually hit. That's why he was bleeding. Also, he said he couldn't feel his right side of his body, because it was in the other dimension. He used the space-time ninjutsu to get himself outta getting killed. Obito is completely capable of using space-time ninjutsu with only 1 sharingan eye(Narutowiki). Tobi also has Senju DNA(he is able to use Izanagi), I'm guessing he got it by mixing himself with a part of Zetsu, who is made from Hashirama's cells. The right portion of Obito's face must have been repaired by Zetsu, thus giving Obito Senju DNA. That is my theory. If you read this whole thing, you're awesome.
PS- I originally thought Tobi was a mixture of people. He was an evil spirit created by all the hate and anger in the shinobi world. That is how he knows everything about a lot of bad people, he could be apart of those ninja.